In a spark plug in which a ground electrode is connected to a metal shell having an external thread and a tapered portion formed on the outer peripheral surface thereof, a spark gap is formed between the ground electrode and a center electrode that is held by the metal shell in an insulated manner. In this type of spark plug, when the external thread of the metal shell is tightened into a threaded hole in a plug hole formed in an engine, a contact portion of the tapered portion is brought into contact with the inner surface of the plug hole. Thus, the ground electrode is positioned about an axis and in an axial direction.
Japanese Unexamined Publication No. 2001-121240 discloses a spark plug processed by cold forging over a range from an external thread of a metal shell to a tapered portion thereof. In the technology disclosed in Japanese Unexamined Publication No. 2001-121240, a contact portion subjected to work hardening by cold forging is difficult to deform. Thus, a ground electrode can easily be positioned by an appropriate tightening torque at a position about an axis where a flow of an air-fuel mixture is not hindered.
In the related art, however, the work hardening is caused by cold forging over the range from the external thread of the metal shell to the tapered portion thereof. Therefore, a portion between the external thread and the contact portion may be cracked and broken by the tightening torque of the external thread.